The present invention relates generally to an exposure apparatus, and more particularly to a so-called immersion exposure apparatus that fills a space between a final lens (final surface) in a projection optical system and a substrate to be exposed with a liquid and exposes the substrate via the projection optical system and the liquid.
A projection exposure apparatus has been conventionally been used to transfer a circuit pattern on a reticle (or a mask) via a projection optical system onto a wafer etc, and an exposure apparatus with a high resolution and a high throughput has recently been increasingly demanded. The immersion exposure attracted people's attentions as one means that satisfies this demand. The immersion exposure promotes a higher numerical aperture (“NA”) of the projection optical system by replacing, with a liquid, a medium between the wafer and the projection optical system. The projection optical system has an NA=n·sin θ where n is a refractive index of the medium, and the NA increases up to n when the filled medium has a refractive index greater than that of air, i.e., n>1. The immersion exposure intends to reduce the resolution R (R=k1(λ/NA)) of the exposure apparatus, where k1 is a process constant and λ is a wavelength of a light source.
A local fill method is proposed for the immersion exposure, which locally fills the liquid in the space between the final lens in the projection optical system and a wafer surface. See, for example, PCT International Publication No. 99/49504. However, it is difficult to fill the narrow space with the liquid, and when the wafer moves at a high speed, the filled liquid disperses. As a result, the filled liquid amount decreases, and air bubbles are mixed in the liquid and cause a diffuse reflection of the exposure light. The diffuse reflection of the exposure light decrease an exposure dose and lower a throughput. One proposed solution for this problem is an air curtain method that sprays a gas around the liquid and maintains the liquid in the narrow space. See, for example, Japanese Patent Application, Publication No. 2004-289126.
However, in the air curtain method, a power of maintaining the liquid to the space is weak if a gas pressure is low, and the liquid is dispersed from the space if the gas pressure is high. Therefore, a control of maintenance of the liquid by using only the air curtain is difficult.